This invention relates to an improved shape for large rolling ingot used in the manufacture of sheet and plate, particularly sheet and plate from light metals such as aluminum and aluminum alloys.
In the conventional manufacture of sheet and plate, a large DC cast rolling ingot having a generally rectangularly shaped transverse cross section is heated to a hot rolling or other elevated temperature and passed several times through a breakdown mill to produce an elongated slab of about 1-2 inches thick. The slab is then passed through a multistand mill while still at an elevated temperature to form a sheet or plate of desired thickness.
When conventional DC cast thick ingots are rolled at elevated temperatures, quite frequently laminations form at the longitudinal edges of the rolled product and these laminations can cause edge cracks during subsequent rolling which must be trimmed. During the initial hot rolling reductions, the top and bottom sections of the ingot are worked extensively, while the center section remains relatively undeformed. During the initial deformation there is a slight lateral spreading and relatively large longitudinal extension of the metal in the top and bottom sections of the ingot. However, because essentially no direct thickness reductions occur to the center section of the ingot during these initial stages of rolling, there is no lateral spreading of the ingot in this section. Indeed, there is usually a slight reduction in the width of the center section of the ingot due to the elongation of the center section caused by the extensions of the top and bottom sections during the early stages of rolling. In the later stages of rolling, the rolling reductions penetrate into the undeformed center section of the ingot, and this section also begins to spread due to these reductions, so that for the remainder of the hot rolling operation the lateral spread of the top center and bottom sections of the ingot are essentially the same. However, because the upper and lower sections of the ingot experience more lateral spread than the center section of the ingot, the over-hanging edges of the top and bottom sections are rolled closer together to ultimately form the laminations previously discussed. The metal in the laminated edges of the rolled slab is heavily worked and under tension and during subsequent rolling is subjected to excessive stresses which cause the formation of cracks. For an excellent discussion of this phenomenon, see Mechanical Metallurgy, 2nd ed. by G. B. Dieter, (1976), pp. 623-628. See, also, article by D. S. Wright et al., in Metals Technology, May 1981, pp. 180-89.
Prior procedures used to minimize edge cracking comprise rolling the edges to extend them and thereby relieve some of the stresses. While these procedures have been effective for the most part in reducing edge cracking, they generate considerable equipment and maintenance problems because one or more sets of vertically oriented rolls must be provided in the mill train. Moreover, edgerolling required on a conventional ingot also increases the amount of liquated structure appearing on the edges of the final rolled surface which must be trimmed. On the latter point it should be noted that usually only the broad faces of the rolling ingot are scalped prior to rolling which removes the liquated structure which forms during casting. The liquated surface on the narrow faces is not removed so if it is rolled onto the surface at the edges it usually must be trimmed.